1. Field of the Invention
This invention relates to apparatus and methods for testing magnetic media. More specifically, the invention relates to apparatus and methods for calibrating test glide head assemblies.
2. Description of the Related Art
The ongoing march toward greater storage densities in the electronics industry has driven the disk drive manufacturers to produce magnetic media having significantly higher areal densities. One of the consequences of increased areal densities is that a single bit of magnetic information occupies a much smaller area on the surface of the disk drive media, requiring that the read/write heads “fly” at a height much closer to the surface. The fly height is on the order of tens of nanometers for today's advanced disk drive designs, and will probably be reduced further in the future. As fly heights decrease, imperfections in the magnetic media surface, particularly those that extend above the plane of the surface, become increasingly important. A collision of the read/write head with an asperity or defect extending above the surface can lead to a catastrophic head crash.
In order to insure proper operation of the disk drive, manufacturers generally test the integrity of the magnetic media by scanning the surface with a test glide head designed to detect contact with defects extending above the magnetic media surface. A specially designed transducer detects contact and produces an electrical signal in response. During media testing, the test glide head flies above the surface at a height lower than the expected fly height of the production disk drive read/write head. This assures no surface media defects will contact the read/write head in actual operation. However, this assurance can only be obtained if the test glide head fly height can be accurately calibrated. This is generally done by using a media disk having asperities of a known dimension deliberately introduced on the surface. The test glide head is then “flown” over the asperities until contact is detected, which then confirms the fly height. It is often desired that the media rotation rate (or linear velocity) be maintained within a target range to assure conformity with the actual operation within the drive. To do this, some other parameter, such as “gram loading” is adjusted to produce contact with the asperities at the desired media RPM. Due to individual structural and aerodynamic differences of each test glide head assembly, the gram loading requirements are different for every assembly, and must be adjusted separately for each. Since each media disk has an upper and a lower surface, two test glide heads are utilized to test both surfaces. These two test heads are often mounted together, opposing each other with the media inserted in between. Due to aerodynamic differences between upper and lower surfaces, the testing must often be done at two different rotation rates, requiring about double the test time to test both upper and lower surfaces.
Typically, the gram loading adjustment is made by measuring the angle of an unloaded suspension and glide head relative to the position the head would have loaded and resting on the disk. See, for example, US Patent Application Publication 2007/0080148. Deflection of the suspension through this angle produces the gram load force on the glide head, which is counterbalanced by hydrodynamic forces when the head is flying above the media surface. In past practice, the unloaded suspension angle is adjusted by thermal treatment of specific suspension components. Similarly, adjustment of glide head pitch angle (pitch static attitude, PSA) and roll angle (roll static attitude, RSA) are made with glide heads in a static condition, that is, not flying over an actual rotating disk surface. The required unloaded suspension angle is correlated by measurement and experience to produce a fly height within a given range, but fine tuning of the RPM is usually necessary to account for individual aerodynamic effects, particularly for test glide heads, where contact with asperities on the calibration disk must produce a response signal within acceptable parameters. The difficulty with this technique is that as fly heights decrease, correlation of static gram loads with actual fly heights becomes more difficult and inaccurate. Furthermore, static adjustment of the upper test glide head and the lower test glide head is not precise enough to ensure both heads will fly at the same height at the same RPM (or linear velocity), requiring that the upper and lower media surfaces be tested separately, doubling test time.
U.S. Pat. No. 6,947,242 discloses an apparatus and method for dynamic fly height and roll adjustment of a physical asperity sensor (PAS) head. The PAS head is used to test disk asperity heights and mechanical interference (commonly known as glide height and take off height). The PAS may be adjusted through a pivoting device such as a joystick coupled through one or more actuators to the pivoting device by actuator arms. In one embodiment, the actuator is a piezoelectric motor. The PAS head may utilize a detector to indicate the distance of the disk asperity from the PAS head. The method comprises receiving the signal and in response, operating the actuators to adjust the position of the pivoting device to obtain a selected positioning of either the fly height or the roll of the PAS.
The difficulty with the forgoing apparatus and method, is that it relies on mechanical actuators for adjustment of the fly height and glide head roll angles. These mechanical actuators must be used with a custom arm assembly that is bulky and expensive, making practical application to current testers difficult. Furthermore, mechanical systems can suffer from the need for frequent calibration due to a multitude of moving parts with tolerances that can change over time.
What is needed is a better method and apparatus for calibrating test glide heads, preferably one that enables the upper and lower surfaces of magnetic media to be tested at the same RPM or linear velocity.